1. Field of the Invention
This invention relates to the field of automatic control of gearshifts in transmissions that occur in accordance with a schedule relating engine throttle position and vehicle speed. It pertains particularly to a control technique that accounts for vehicle acceleration and throttle speed to minimize gearshift delay and to produce gearshifts at the correct scheduled vehicle speed.
2. Description of the Prior Art
The invention relates to automotive transmission controls of the type disclosed in U.S. Pat. No. 4,943,921 and U.S. Ser. No. 289,889, filed Dec. 27, 1988. That application and patent are assigned to the assignee of this invention.
In an electronically controlled, variable speed automatic transmission, the current gear ratio is determined on the basis of a schedule, stored in electronic memory accessible to a microprocessor, relating vehicle speed to throttle position. The schedule defines boundaries between operating conditions on the basis of vehicle speed and throttle position, each boundary separating an operating range for a particular gear ratio from an adjacent range of these variables where a different gear ratio is the appropriate gear ratio. Generally for a current gear ratio, the shift schedule produces a reference vehicle speed, the dependent variable, when memory is addressed with the current throttle position, the independent variable. The reference vehicle speed is compared to current vehicle speed periodically so that upshifts and downshifts from the current gear occur when vehicle speed rises above or falls below boundaries corresponding to the current gear ratio and throttle position.
However, a typical transmission requires several seconds to complete a gearshift following issuance of a signal commanding the shift, the delay resulting principally from time required to energize and deenergize solenoid-operated valves in a hydraulic control circuit, response time to move hydraulic shift valves in the circuit, and response time to apply and release hydraulic servos that alter the state of clutches and brakes in the gearbox whose engagement produces the new gear ratio.
Normally the vehicle is accelerating when a gearshift command issues. Although the shift is commanded when current vehicle speed exceeds the scheduled value corresponding to the current throttle position and current gear ratio, yet the shift is completed at a different vehicle speed due to transmission delay (usually between one and two seconds) and vehicle acceleration during the delays period. Consequently upshifts and downshifts can occur too late.
The throttle is opened by depressing an accelerator pedal. Typically, this action produces a downshift to a lower operating gear ratio where greater torque is delivered to the drive wheels of the vehicle. Consequently, vehicle speed increases. But if vehicle speed exceeds the intended vehicle speed, the accelerator pedal is partially released, thereby inducing an upshift to a higher gear ratio where drive wheel torque decreases and vehicle speed drops.
Operation of the transmission in combination with the engine throttle in this objectionable manner is called "hunting", a cyclic repetition of gear shifts between adjacent gear ratios with corresponding changes in vehicle speed.
A conventional technique to cure delayed upshifts and downshifts is to relocate the shift calibration points to compensate for the delay error at a typical vehicle acceleration. This action produces shift timing errors at all other accelerations and may be impossible to implement if it causes upshift and downshift calibration lines to intersect.
Various techniques have been devised to avoid shift delay error. A control technique that uses throttle speed to define a new gearshift schedule is described in U.S. Pat. No. 4,490,790. The control accelerates the vehicle rapidly when throttle rate is high and slowly when throttle rate is low. There is no provision for vehicle acceleration to reduce transmission delay to the expectations of the driver.
U.S. Pat. No. 4,335,428 describes a technique for detecting increased engine load when running on a slope and determining the appropriate gear ratio in accordance with the magnitude of the loading associated with the inclination of the slope. Slope detecting reference data, indicative of vehicle speeds for various ranges of slope correspond to throttle openings in different gear ratios, are used to detect presence of the slope by comparing the actual gear ratio, the throttle opening, vehicle speed, against the slope detecting reference data.
Signals indicative of the shift lever position, vehicle speed and throttle opening are applied as input to a microprocessor, a slope detecting program determines presence of the slope from these data and modifies standard shift data to produce shift reference data, which is then written into memory. By comparing the current gear ratio throttle opening and vehicle speed against corresponding reference data, a speed ratio is determined and used to energize solenoid valves to produce gear ratio changes in accordance with the control programs.
The gear shift control of U.S. Pat. No. 4,285,252 uses a stored schedule of vehicle acceleration and a comparison between actual vehicle acceleration to the stored value to change scheduled gear shift in accordance with differences between stored and actual vehicle acceleration. The control includes a first schedule of gear shift changes suitable when the vehicle is traveling on a flat road and a second schedule for gear changes suitable for an uphill road. On the basis of a stored schedule of vehicle accelerations, reference to throttle position and actual vehicle acceleration, the slope of the grade is determined, and a gear shift schedule corresponding to the slope is selected to control gear shifts while on the grade.
The gear shift control of U.S. Pat. No. 4,380,048 relies on an average value of engine load and vehicle speed to determine a programmed acceleration, which is compared to actual acceleration to determine whether a transmission shift characteristic is to be selected.
The gear shift control of U.S. Pat. No. 4,897,790 intentionally skips selected drive ratios on the basis of perceived driving conditions and modifies the scheduled gear ratio change on the basis of vehicle acceleration.
It is preferable that a gearshift control for automotive use should anticipate the effect of vehicle acceleration and inherent delay and command each gearshift before the shift is required. If transmission delay is determined for all possible gearshifts, that delay can be compensated accurately on the basis of vehicle acceleration so that response to the shift command produces a change of gear ratio at the scheduled vehicle speed. Account can be taken also for the rate of throttle position change to further advance the gearshift command in accordance with driver expectations as evidenced by control of the accelerator pedal.